1. Field of the Invention
The present invention relates to a technique for checking a measurement result more accurately in a jitter measuring apparatus which measures a jitter characteristic of a digital signal output from a transmitter used for the high-speed digital communication represented by SDH (Synchronous Digital Hierarchy) and which displays the measurement result.
2. Description of the Related Art
In a system which transmits a binary digital signal, deterioration of the signal quality caused by a phase change in the digital signal, that is, caused by jitter is a problem.
For this reason, a jitter of a digital signal output from an object to be measured, such as a transmitter which handles a digital signal, is measured in order to measure the quality of the digital signal output from the object to be measured, the resistance to jitter, a transmission characteristic, and the like.
FIG. 6 shows the configuration of a known jitter measuring apparatus 10 used for the above-described object.
The jitter measuring apparatus 10 includes: a jitter demodulator 11 which receives a digital signal Dr of an object to be measured and demodulates a jitter component (phase-modulated component); a jitter amount detector 12 which detects as a jitter amount the amplitude, such as an effective value (RMS) or a peak-to-peak value, of a demodulation signal (assumed to be a signal converted into the digital value) output from the jitter demodulator 11; and a display unit 13 which displays the detected jitter amount as a numerical value.
In addition, a wideband jitter demodulator, a filter circuit of a band based on ITU-T recommended 0.172, and an A/D converter which operates at a predetermined sampling rate for outputting a demodulation signal as a digital value are included in the jitter demodulator 11.
An example of the jitter measuring apparatus which measures the jitter of an input digital signal as described above is disclosed in JP-A-2001-133492.
In the known apparatus described above, however, the waveform of a demodulation signal cannot be observed. Accordingly, in the case of a waveform which is completely different from a waveform that a measuring person expects, it is difficult to grasp the situation from the numerical result.
In order to solve such a problem, functions of converting the output of the jitter demodulator 11 into an analog signal by a D/A converter 15, inputting the converted analog signal to a filter 16 to remove a high-frequency component generated in the D/A conversion processing, and outputting it to the outside are provided in a known apparatus as shown by the dotted line in FIG. 6.
In the known apparatus, therefore, the waveform of a demodulation signal can be observed by inputting an analog demodulation signal, which is output to the outside, to an oscilloscope.
However, when the waveform of the analog demodulation signal output to the outside is observed as described above, the amplitude of the displayed wave may be largely different from the value detected by the jitter amount detector 12 due to the performance of the D/A converter 15 and the filter 16.
Particularly, the peak-to-peak value is largely affected by the analog characteristics of the D/A converter 15 and the filter 16. Accordingly, since the peak-to-peak value is different from the value detected at the main apparatus side in many cases, a decrease in accuracy on a screen for waveform observation becomes a problem. For this reason, it could not be said that the known apparatus performed sufficiently well when more accurate waveform information was required in the field of research and development of a transmitter and the like.
In addition, since the oscilloscope should be prepared to observe the waveform of a demodulation signal, there was also a problem that the installation place became large or a problem that the cost increased.